Shape steel material having high strength, high toughness and excellent fire resistance and process for producing rolled shape steel of said material

Abstract

After a predeoxidation treatment of a molten steel comprising as basic ingredients, in terms of % by weight, 0.04 to 0.20% of C, 0.05 to 0.50% of Si, 0.4 to 2.0% of Mn, 0.3 to 0.7% of Mo, 0.003 to 0.015% of N, 0.04 to 0.20% of V and less than 0.005% of Al to regulate [O %] to 0.003 to 0.015% by weight, titanium is added thereto so as to satisfy a requirement represented by the formula: -0.006</=[Ti %]-2[O %]</=0.008 to crystallize a titanium-based oxide in an amount of 20 particles/mm2 or more, and MnS, TiN and V(C, N) are deposited on the titanium-based oxide to disperse the titanium-based oxide as a composite precipitate in the steel to provide a cast slab, and a high-strength high-toughness shape steel having an excellent fire resistance is provided by a combination of water cooling between rolling passes at the time of rolling with accelerated cooling after the completion of the rolling.

We claim: 
     
       1. A cast slab produced by subjecting a molten steel consisting essentially of, in terms of % by weight, 0.04 to 0.20% of C, 0.05 to 0.50% of Si, 0.4 to 2.0% of Mn, 0.3 to 0.7% of Mo, 0.003 to 0.015% of N, 0.04 to 0.20% of V and less than 0.005% of Al with the balance consisting of Fe and unavoidable impurities, to a predeoxidation treatment to regulate the dissolved oxygen concentration to 0.003 to 0.015% by weight, adding titanium so as to produce a titanium content of 0.005 to 0.025% by weight and to satisfy a requirement of the relationship between the titanium content [Ti %] and the dissolved oxygen concentration [O %], represented by the formula: -0.006≦[Ti %]-2[O %]≦0.008, to crystallize a titanium-based oxide in an amount of 20 particles/mm 2  or more and depositing MnS, TiN and V(C, N) on the titanium-based oxide, during cooling, to disperse the titanium-based oxide as a composite precipitate in the steel. 
     
     
       2. A cast slab produced by subjecting a molten steel consisting essentially of, in terms of % by weight, 0.04 to 0.20% of C, 0.05 to 0.50% of Si, 0.4 to 2.0% of Mn, 0.3 to 0.7% of Mo, 0.003 to 0.015% of N, 0.04 to 0.20% of V and less than 0.005% of Al and further consisting essentially of at least one member selected from 0.7% or less of Cr, 0.05% or less of Nb, 1.0% or less of Ni, 1.0% or less of Cu, 0.003% or less of Ca and 0.010% or less of REM with the balance consisting of Fe and unavoidable impurities, to a predeoxidation treatment to regulate the dissolved oxygen concentration to 0.003 to 0.015% by weight, adding titanium so as to produce a titanium content of 0.005 to 0.025% by weight and to satisfy a requirement of the relationship between the titanium content [Ti %] and the dissolved oxygen concentration [O %], represented by the formula: -0.006≦[Ti %]2[O %]≦0.008, to crystallize a titanium-based oxide in an amount of 20 particles/mm 2  or more and depositing MnS, TiN and V(C, N) on the titanium-based oxide, during cooling to disperse the titanium-based oxide as a composite precipitate in the steel.